2019 AIME II Problem 12
Below is the professionally curated solution for Problem 12 of the 2019 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2019 AIME II solutions, or check the answer key.
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Difficulty rating: 3060
12.
For call a finite sequence of positive integers progressive if and divides for Find the number of progressive sequences such that the sum of the terms in the sequence is equal to
Solution:
Divisibility is transitive, so every term of a progressive sequence is a multiple of the first term. If a sequence with sum has length at least and first term then and dividing the remaining terms by yields a progressive sequence with first term at least and sum this correspondence is reversible. So if denotes the number of progressive sequences with sum and first term at least the answer is the leading counting the sequence
The same reduction gives the recursion with in particular when is prime. Working upward:
The divisors give arguments whose -values are summing to Adding the single-term sequence gives
Problem 12 in Other Years
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